The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
For a buck converter and any variants, e.g. a forward converter or a bridge converter, the operating duty cycle (D), when in continuous inductor current mode is defined as:D=Vout÷Vin  (1)where Vout is the output voltage of the converter and Vin is the input voltage to the converter.
For a power converter required to operate over a range of trim able output voltages, the operating duty cycle becomes very small when output voltage is set very low with respect to a nominal output.
Some current source applications demand a very low current setting compared to a nominal rating or sometimes a load impedance is unusually low. These conditions also force the output voltage to be low.
In operation a power converter has a minimum controllable operating on-time. The minimum operating on-time is affected by the resolution of a controller's pulse width modulator (PWM). However, known digital controllers have PWM resolutions down to an order of a few hundred pico-seconds and thus for most applications this will not be a design limitation. Of course, for analog control, PWM resolution is not an issue.
However, external factors such as propagation delays in circuits, buffers, drivers, and turn-on and turn-off delays in power mosfets do create a minimum controllable on-time before the power converter is forced into burst mode. Once the minimum on-time is reached and reliable operation control is lost energy balancing is used in prior art control schemes. Typically energy balancing includes supplying excess energy by operating at duty cycles above what is actually needed followed by turning the converter off to deliver a desired average effective power. During energy balancing, the control loop may be excessively slow with a converter output that has excessive ripple. The slow control loop response and excessive ripple out is unacceptable for many applications. Some examples requiring faster control loop response and low ripple include magnet drivers and super magnet where the current must pass through zero smoothly from positive to negative polarity for four quadrant operation. Another example is lab instrumentation or bench power supplies requiring output voltage and currents to be adjustable from a near zero value to a predetermined maximum value. The prior art met such needs via operation at low switching frequencies, resulting in bulky power supply designs or a linear controller is used at the cost of low power conversion efficiency. Still another example is high current multi-phase buck converters used in voltage regulator modules (VRMs) for computing and other applications. VRMs typically have a significant drop-off in efficiency as the output voltage is lowered for a given load current. It is possible to design separate VRMs for each required output to avoid the efficiency drop-off. However, VRMs have become commodity products and configurability is very important to keep manufacturing costs down by allowing for fewer SKUs (stock keeping units). Therefore, the inventors have recognized a need to maintain continuous mode and prevent the converter from being forced into burst mode at any of low output voltages, low output current, or low load relative to nominal conditions while still providing low ripple output and sufficient control loop response.